The present invention relates to an active matrix addressing type liquid crystal display device using thin film transistors (TFTs) and a method of manufacturing the same.
An active matrix addressing type liquid crystal display device includes switching elements each being provided correspondingly to one of a plurality of pixel electrodes arranged in a matrix. The active matrix addressing type display has a feature that liquid crystal at each pixel is theoretically driven at all times and accordingly it is higher in contrast than a simple matrix type adopting time-multiplexed driving. Such an active matrix addressing type display has become essential, particularly for color display.
A TFT arrangement used for a conventional active matrix addressing type liquid crystal display device includes a scanning signal line (gate line) formed on a transparent insulating substrate; a gate insulator formed on the scanning signal line; a semiconductor layer formed on the gate insulator; and a drain electrode (data line) and a source electrode formed on the semiconductor film, wherein the source electrode is connected to a transparent pixel electrode and the drain electrode (data line) is supplied with a video signal voltage. A TFT structure of a type in which a gate electrode is formed directly on a substrate is generally called an inverted staggered structure. Such a TFT is known from Japanese Patent Laid-open No. Sho 61-161764.
The liquid crystal display device using TFTs enables active addressing and thereby it exhibits high contrast; however, it is complicated in formation of TFTs on a substrate and also requires six or more photolithography steps. This is disadvantageous in terms of increasing manufacturing cost of a TFT substrate and decreasing processing yield with the increased number of manufacturing steps due to dust or dirt.
A method for simplifying manufacturing steps has been proposed, wherein a gate insulator, a semiconductor layer, and a metal film (for drain and source electrodes) are formed; the semiconductor layer is processed using the metal film as a mask; and a transparent electrode is formed. This prior art, however, is disadvantageous in that in the case where the metal film forming the source electrode is smaller in etching rate than the semiconductor film, the source electrode overhangs and increases open line defect probabilities for the transparent electrode due to the presence of a step at the overhang. In other words, in the above prior art the manufacturing yield has never been considered sufficiently.
It is required to increase the size of an area of a transmitting portion (hereinafter, referred to as an aperture ratio) of a transparent pixel electrode for realizing a bright display screen. The above-described prior art, however, failed to improve the aperture ratio for obtaining a display of high contrast and low cross talk.